1. Technical Field of the Invention
The present invention relates to an interface circuit for a telecommunications line, and particularly to a hybrid circuit for communicating over a telephone line.
2. Background of the Invention
Hybrid circuits have been used in modems or other communication devices to isolate the modem receiver from being affected by transmissions of the modem transmitter. Known hybrid circuits include a transformer circuit utilizing at least two transformers. Although somewhat effective in isolating the modem receiver path from the modem transmitter path, the transformer hybrid circuit is expensive to implement on an integrated circuit.
A second known hybrid circuit is a resistive divider circuit implemented in a resistor bridge configuration. The resistive divider circuit, however, degrades the signal strength of the signal transmitted over the telecommunications line and itself generates an appreciable amount of thermal noise due to the presence of resistors therein, thereby providing a relatively poor signal-to-noise ratio. Consequently, for handling communication over a relative high loss line, such as a twisted pair telephone line, a resistive divider hybrid circuit is not a perfect solution.
Other attempts to separate the receive path of a modem from the transmit path thereof include the use of a filter. In order to provide a substantially complete isolation of the received signals from the transmitted signals, however, a very high order filter having a relatively high dynamic range is required. As is well known, such filters are quite expensive to implement.
Based upon the foregoing, there exists a need for a hybrid circuit for a modem or other device which effectively and economically isolates the receive path from the transmit path.